1. Field of the Invention
The present invention relates to a linear power amplifying apparatus, and more particularly, to a linear power amplifying apparatus in base station for a mobile communication.
2. Description of the Background Art
Generally, a radio frequency band amplifier using active elements creates undesired distorted components according to power level variation. Especially, when an amplifier is operated in the vicinity of a saturation area, a non-linear phenomenon remarkably occurs, distorting an amplitude and a phase of an output. In case that more than two signals are inputted, inter-modulation distortion (IMD) components are generated between the inputted signals, having a big influence on an adjacent channel. Such IMD components work as a noise source, degrading a transfer quality of a communication system.
Accordingly, in case of an amplifier in use for a radio frequency transmission terminal of the base station for mobile communication, an operating point of the amplifier is positioned at a few decibel (dB) back-off position from the saturation area, so that the amplifier can be operated at the linear area.
In this case, however, since the input level of the amplifier needs to be low, so that its output level becomes too low.
In an effort to solve the problem, a research has been conducted for a method to add a linear circuit which minimizes an IMD component among outputs of the amplifier, to the amplifier. This method is advantageous in that even though the amplifier is operated in the non-linear area in the vicinity of the saturation area, an output signal of a desired amplitude having less IMD component is obtained.
FIG. 1 is a schematic block diagram of a linear power amplifying apparatus including a linear circuit in accordance with a conventional art.
As shown in the drawing, a linear power amplifying apparatus includes a pre-amplifier 1, a first distributor 2, a main amplification circuit 6, a second distributor 7, a second delay unit 8, a second coupler 9, a first delay unit 10, a first coupler 11 and an error amplification circuit 15.
The pre-amplifier 1 amplifies an infinitesimal input signal to a predetermined amplitude with which main amplification circuit 6 is operable. The first distributor 2 distributes the output signal of the pre-amplifier 1.
The main amplification circuit 6 amplifies one signal distributed by the first distributor 2 to an amplitude suitable to a mobile communication radio interface. The second distributor 7 distributes the amplified signal outputted from the main amplification circuit 6.
The first delay unit 10 delays the other signal distributed by the first distributor 2 as long as the time taken for one signal to pass the main amplification circuit 6. The first coupler 11 couples one signal distributed by the second distributor 7 and the other signal delayed by the first delay unit 10.
The error amplification circuit 15 amplifies an error signal outputted from the first coupler 11 as large as the amplification of the main amplification circuit 6. The second delay unit 8 delays the other signal distributed by the second distributor 7 as long as the time taken for one signal to pass the error amplification circuit 15.
The second coupler 9 couples the signal delayed by the second delay unit 8 and the signal outputted from the error amplification circuit 15 and outputs it.
The main amplification circuit 6 and the error amplification circuit 15 includes variable attenuators 3 and 12 for variably attenuating an inputted signal, variable phase shifters 4 and 13 for varying a phase of a signal outputted from the variable attenuators 3 and 12, and a main amplifier 5 and an error amplifier 14 each for amplifying the signal outputted from the variable phase shifters 4 and 13 to a predetermined amplitude, respectively.
The operation of the conventional linear power amplifying apparatus constructed as described will now be explained.
First, when an infinitesimal input signal is inputted, the pre-amplifier 1 amplifies the input signal to a predetermined amplitude with which the main amplification circuit 6 is operable.
The first distributor 2 distributes the output signal of the pre-amplifier 1 and outputs one signal to the main amplification circuit 6 and the other signal to the first delay unit 10.
The signal outputted to the main amplification circuit 6 passes the variable attenuator 3, the variable phase shifter 4 and the main amplifier 5, and then is amplified to an amplitude suitable to the mobile communication radio interface. At this time, a large amount of harmonics are generated.
The second distributor 7 distributes the output signal including the harmonics component from the main amplification circuit 6, and outputs one signal to the second delay unit 8 and the other signal to the first coupler 11.
Meanwhile, the first delay unit 10 the signal distributed by the first distributor 2 as long as the passing time of the main amplification circuit 6 and outputs it to the first coupler 11.
The first coupler 11 couples the signal including the harmonic component distributed by the second distributor 7 and the signal delayed by the first delay unit 10 and outputs a carrier-cancelled error signal.
The error amplification circuit 15 amplifies the error signal outputted from the first coupler 11, passing through the variable attenuator 12, the variable phase shifter 13 and the error amplifier 14, and amplifies the amplitude of amplification of the main amplification circuit 6. At this time, the error amplifier 14 amplifies only the amplitude without generating harmonics.
The second delay unit 8 delays the signal including the harmonics outputted from the second distributor 7 as long as the passing time of the error amplification circuit 15 and outputs it to the second coupler 9.
The second coupler 9 couples the carrier signal including the harmonics received from the second delay unit 8 and the error signal of which only the harmonics has been amplified received from the error amplification circuit.
At this time, the carrier signal including the harmonics and the error signal of which only the harmonics has been amplified have the same amplitude to each other and are adjusted to have a phase difference of 180xc2x0. Thus, when the two signals are coupled by the second coupler 9, the high frequency component are mutually canceled, so that an amplified carrier signal having an amplitude suitable to the mobile communication radio interface, which is similar to the original input signal, is outputted.
However, the conventional linear power amplifying apparatus has a problem that the variable attenuator which variably attenuates the amplitude of the input signal and the variable phase shifter which variably shifts the phase of the input signal are not accurately controlled, resulting in that it is difficult to obtain a signal having the opposite phase, and thus, the harmonic component cancellation effect is degraded.
Therefore, an object of the present invention is to provide a linear power amplifying apparatus suitable to cancellation of harmonics in a mobile communication.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a linear power amplifying apparatus including: a first distributor for distributing an input signal; a variable attenuator for receiving one signal distributed by the first distributor and variably attenuating the amplitude of the signal according to an amplitude control signal; a variable phase shifter for shifting a phase of the signal outputted from the variable attenuator according to a phase control signal; a main amplifier for amplifying the signal shifted by the variable phase shifter to a predetermined level; a second distributor for distributing the signal outputted from the main amplifier; a delay unit for delaying the other signal distributed by the first distributor for a predetermined time and outputting it; and a carrier cancellation loop circuit for comparing the amplitude and phase of the output signal of the delay unit and those of the output signal of the second distributor, and generating the amplitude control signal for compensating an amplitude difference and a phase control signal for compensating the phase difference between the two output signals, respectively.
In order to achieve the above objects, there is also provided a linear power amplifying apparatus including: a first distributor for distributing the input signal; a first variable attenuator for receiving one of signals distributed by the first distributor and variably attenuating the amplitude of the signal according to a first amplitude control signal; a first variable phase shifter for shifting the phase of the signal outputted from the first variable attenuator according to a first phase control signal; a main amplifier for amplifying the signal shifted by the variable phase shifter to a predetermined level; a second distributor for distributing the signal outputted from the main amplifier; a first delay unit for delaying the other signal distributed by the first distributor for a predetermined time and outputting it; a carrier cancellation loop circuit for comparing the amplitude and phase of the output signal of the first delay unit and those of one output signal distributed by the second distributor, and generating the first amplitude control signal for compensating an amplitude difference and the first phase control signal for compensating a phase difference between the two output signals, respectively; a second variable attenuator for receiving an error signal from the carrier cancellation loop circuit and attenuating the amplitude of the signal according to a second amplitude control signal; a second variable phase shifter for receiving a signal attenuated by the second variable attenuator and shifting the phase of the signal according to a second phase control signal; an error amplifier for amplifying the error signal outputted from the second variable phase shifter without harmonics; a second delay unit for delaying the other output signal of the second distributor for a predetermined time and outputting it; and an error cancellation loop circuit for comparing the amplitude and the phase of the output signal of the second delay unit and those of the error signal outputted from the error amplifier, and generating the second amplitude control signal for compensating an amplitude difference and the second phase control signal for compensating a phase difference between the output signal of the second delay unit and the error signal, respectively.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.